Coarse sea spray inhibits lightning

The known effects of thermodynamics and aerosols can well explain the thunderstorm activity over land, but fail over oceans. Here, tracking the full lifecycle of tropical deep convective cloud clusters shows that adding fine aerosols significantly increases the lightning density for a given rainfall amount over both ocean and land. In contrast, adding coarse sea salt (dry radius > 1 μm), known as sea spray, weakens the cloud vigor and lightning by producing fewer but larger cloud drops, which accelerate warm rain at the expense of mixed-phase precipitation. Adding coarse sea spray can reduce the lightning by 90% regardless of fine aerosol loading. These findings reconcile long outstanding questions about the differences between continental and marine thunderstorms, and help to understand lightning and underlying aerosol-cloud-precipitation interaction mechanisms and their climatic effects.

Effect of meteorology on aerosol-driven lightning variation. To further isolate the effects of meteorology and aerosols on lightning, other relevant meteorological factors are examined referring to the previous study 1, 2 , including 450 hPa vertical velocity (ω), relative humidity (RH) and 850-200 hPa wind shear (WSH). The results show that lightning significantly increases with precipitable water (PW), convective available potential energy (CAPE) and 450 hPa updraft (blue to red lines), while lightning decrease with the increase in 850-200 hPa wind shear ( Supplementary   Fig. 7). These are consistent with our previous study 2 , which indicates that convection becomes stronger (higher cloud top, longer lifetime and more rainfall amount) under the condition of higher PW, CAPE and updraft, but lower wind shear.
The lightning increase with higher surface RH over land (R=0.34), while it is opposite over ocean, but with very low correlation (R=-0.11). The low correlation over ocean is possibly attributed to the abundant and homogeneous water vapor (RH>70%) at surface over ocean ( Supplementary Fig. 8a), resulting in the effect of surface RH is not highlighted. However, the consistently positive relationships are found between lightning and RH at the middle troposphere (450 hPa), where RH distribution is similar over both land and ocean. Therefore, aerosol-driven lightning variations for fine aerosol and coarse sea salt are consistent and significant for whatever meteorology bins ( Supplementary Figs. 1 to 7). This indicates that the relationships between aerosols and lightning are general and independent on meteorology.
Isolating the effect of wind speed and coarse sea salt. Additionally, the coarse sea salt concentration is directly driven by the near-surface wind speed over ocean 3  To isolate the mixed effects of fine aerosol and coarse sea salt on lightning, we further discuss the aerosol-driven lightning variations with fine aerosol (coarse sea salt) at fixed other aerosol and wind speed. With the increase of fine aerosol from clean to polluted condition, the lightning density for unit rainfall amount increase by an average of one order of magnitude ( Supplementary Figs. 10a-10c). This is independent on coarse sea salt and wind speed, except for some change in magnitude. Additionally, after isolating the disturbance of fine aerosol and wind speed, there is still significant inhibition on lightning with additional coarse sea salt.
In general, the lightning significantly increases with fine aerosol, especially over ocean.
However, a comparable significant decrease in lightning is driven by coarse sea salt by enhancing warm rain over ocean. These aerosol-driven lightning variations are consistent and significant for whatever meteorology bins. This indicates that the relationships between two aerosols and lightning are general, comparable and independent. (GOES16/17 GLM). Thus, the light must exit the top of the cloud with enough intensity to be detected from space. This causes these detectors regularly miss a significant number of cloud-to-ground strokes, including. Because much of these light exits below the cloud and propagates out parallel to the surface, and is missed by the satellite.
Furthermore, the actual total strokes located in the region of interest are seen only by the low earth orbiters, such as TRMM/LIS and ISS/LIS, but not by the GOES16/17 satellites. However, the actual total strokes for DCC detected by the low earth orbiters are underestimated by 1 or 2 orders of magnitude because of the low temporal resolution of these low earth orbiters 5  Detection efficiency of WWLLN. The relative detection efficiency (RDE) is a measure of how well a given location in the network is being observed relative to the best region in the network.
The global RDF of WWLLN is published every day (http://wwlln.net/deMaps). In general, the RDF for strokes is near 100% everywhere, with the lowest relative detection efficiency down to 75% (i.e., Antarctica and parts of Africa and eastern Asia) 6 . Additionally, lightning sferics propagate with very low attenuation over the oceans, and thus lightning is detected over water even better than over land 5 . Note that there is scarce global 'ground truth' data set against which the absolute detection efficiency of WWLLN can be determined presently, especially over ocean.
The latest compilation of WWLLN absolute detection efficiency is given by presented in study of Also, the WWLLN is more efficient at locating the stronger lightning strokes. Hutchins,et al. 6 showed that the distribution of stroke energies is log-normal within 1 hour, meaning that there is a normal distribution of energies, which would include some large strokes and some smaller ones.
Therefore, for a weak storm with just one small stroke, WWLLN may not have located it.
However, for a large number of strokes (i.e. strong storm), WWLLN is likely to have identified strokes from every thunderstorm. Actually, WWLLN was locating nearly all synoptic thunderstorms in 2006, as shown in the study of Jacobson, et al. 8 . Additionally, Supplementary   Fig. 12 shows the normalized probability distributions for energy per lightning of tracked DCCs in this study. It shows that lightning intensity distribution over land and ocean are statistically similar. Therefore, the similar lightning intensity distributions support a consistent detection efficiency for the total lightning over both land and ocean.
Possible influence of lightning data on the results. Holzworth, et al. 7 show that WWLLN detects 20% of the strokes at 15 kA, and this ratio is about 70-80% for all strokes. Previous works prove that WWLLN has better detection on lightning over ocean compared to ground-based and TRMM/LIS observations 5,7 . This indicates that WWLLN was locating nearly all synoptic thunderstorms 8 , while it is possibly underestimating the weak strokes, especially over land.
Moreover, there is still consistent land-ocean contrast of lightning density based on WWLLN.
Therefore, WWLLN lightning data support our conclusion about the effects of fine and coarse sea salt aerosols over ocean, and the cause of land-ocean contrast of lightning. Actually, the aerosol effect on lightning measured by WWLLN has been compared to that measured by the completely independent TRMM LIS satellite optical sensor over the shipping lanes and found a similar response to the ship emissions 9 . In summary, the results we present in this paper are not likely to be dominated by WWLLN biases. Supplementary Table 1 | Statistics of aerosol invigoration for deep concective cloud (DCC) properties and lightning at the 3×3 fixed precipitable water (PW) and convective available potential energy (CAPE). ∆CTT (cloud top temperature) indicates the change of CTT of core from clean to the optimal condition of fine aerosol over land and ocean. The number of lifetime, rainfall amount, lightning density and normalized lightning density indicate the enhancement factor from clean to the optimal condition of fine aerosol. The data was binned into three approximately equal size classes on PW and CAPE over land and ocean, respectively.